Transparent conductive systems (conductive substrates) are needed, by way of example, for production of modern displays or solar cells. Other applications are screening with respect to electromagnetic radiation and general use as electrode material.
Frequently used substrates are glass and similar materials. However, flexible substrates, such as polyester film, are desirable for many applications.
In industry, films having a conductive coating are generally metallized films. These films generally have low transparency (<30%) for visible light, and have to be metallized in a vacuum process. Transparent conductive films often have a coating with ITO (indium tin oxide) or ATO (antimony tin oxide) applied via sputtering (see WO 2003/12799 from Toyo Boseki). However, the sputtering process limits production rate and the dimension of the item to be coated and is therefore uneconomic. It is therefore desirable to be able to coat the film by a coating method which permits higher production rates. An example of this type of method is coating from solution or dispersion, inter alia by means of application rolls. For this, the conductive constituents have to be introduced into a coating solution. Conductive coatings may, by way of example, comprise conductive, organic polymers.
JP-A-2000-188432 (TDK Corp., JP) and JP-A-2001-099924 (Sekisui Chem. Corp., JP) describe electrically conductive coatings based on nano-scale ITO and, respectively, ATO. However, the conductive coatings described there have the disadvantage of themselves having low flexibility, due to the brittleness of the ITO coating, therefore losing their conductivity when exposed to mechanical stress, such as flexure.
JP-C-2003-154594 and EP-B-1 020 762 describe coatings with low conductivity based on organic conductive components in the range from 107 to 109 ohm/cm2. Among the disadvantages here are the low conductivities described and the low stability of the conductive materials used with respect to oxidative degradation and UV light (below 380 nm).
EP-B-0 962 486 describes coatings with low conductivity based on organic conductive components in the range from 108 to 1010 ohm/cm2. Here, conductive inorganic metal oxides are mixed with conductive organic polymers and coated onto polyester film. Among the disadvantages here are the low conductivities described, and no information is given concerning the transparency of the films. Reasons for the low performance level of the films described may well lie within the method of coating, which takes place on a polyester film which has not at that stage completed its crystallization, and indeed the film is then oriented and thus exposed to high mechanical stress which, in particular when inorganic conductors are used, is associated with embrittlement of the layer with loss of most of the conductivity and of the transparency.